Generally, in a radar system, the direction in which a target is present can be obtained by calculating the pointing direction of a beam whose received signal is detectable. The narrower the beam width, the more enhanced the accuracy of measuring the direction of the target. However, when trying to narrow the beam width while keeping the wavelength of a transmission wave of the radar constant, an antenna having a larger aperture diameter is required. Therefore, instead of a method in which the measurement accuracy depends on the beam width, a method of measuring target directions using information such as differences in amplitude or phase of received signals obtained from a plurality of beams whose directions are slightly different from each other has been conventionally used. According to such a method, an angular resolution higher than the measurement accuracy determined from the beam width can be achieved.
As examples of such a method, a sequential-lobing system and a monopulse system have been publicly known. In these methods, firstly, two beams whose directions are adjacent to each other are selected from a plurality of directions of beams. And the difference (referred to as a Δ signal) in, and the sum (Σ signal) of, the amplitudes or phases of received signals observed from the two beams are calculated. Next, the ratio of the Δ signal and the Σ signal is calculated. Assuming that the ratio is referred to as a Δ/Σ value, the Δ/Σ value uniquely corresponds to the angle of the target, so that the direction of the target can be estimated from the Δ/Σ value.
In these methods, however, the number of targets is limited to one. More specifically, there is a problem in that, when a plurality of targets is present within the same beam, the directions cannot be accurately calculated. As illustrated in FIG. 17 for example, considering a radar system for estimating the direction of a target 110 using beams radiated from an antenna 100 to directions 101 through 105 (assuming that the directions 101 and 102, the directions 102 and 103, the directions 103 and 104, and the directions 104 and 105 are respectively adjacent to each other, and the beams to those directions are referred to as beams 101 through 105), regardless of whether or not a true target is present, some sort of angle value would be calculated from each of combinations of a beam 1 and a beam 2, the beam 2 and a beam 3, the beam 3 and a beam 4, and the beam 4 and a beam 5, which are adjacent to each other. Arrows 111, 112, 113, and 114 illustrated in FIG. 18 are examples of the directions of the angle values (images) calculated based on differences in received signals obtained from the beams 101 through 105 in FIG. 17. In the figure, the direction 111 has been calculated from the combination of the beams 101 and 102, and the direction 112 has been calculated from the combination of the beams 102 and 103. In addition, the direction 113 has been calculated from the combination of the beams 103 and 104, and the direction 114 has been calculated from the combination of the beams 104 and 105. Although the direction 113 and the direction 114 are associated with reception waves that result from transmission waves in the beams 103 through 105 radiated toward the target 110 and reflected thereby, the direction 111 and the direction 112 are not associated with the target 110, and are false images that do not correspond to true targets.
Here, when there is only a single target, false images can be rejected based on the amplitude or power of the received signal. However, when there are two or more targets, correlations between the targets and the directions obtained from combinations of beams become complicated, so that simple rejection based on thresholds is not applicable.
As a method of measuring directions of a plurality of targets, a maximum likelihood estimation method (maximum likelihood localization) is disclosed in “Maximum likelihood localization of multiple sources by alternating projection” by I. Ziskind and M. Wax, IEEE Transaction on Acoustics Speech and Signal Processing, vol. 36, no. 10, pp. 1553-1560, October 1988. According to the method, separation of directions is possible even if a plurality of targets is present within a beam. However, this method needs a lot of computation amount, and requires a signal processing unit having a high computing power. In particular, the more the number of targets, the more the computation amount.
As described above, problems have been that the sequential-lobing system and the monopulse system cannot separate the directions of a plurality of targets, and that the maximum likelihood estimation method can separate the directions of a plurality of targets, but computing load is high.
The present invention aims to resolve above-described problems in the existing methods of calculating target directions by combining a plurality of beams.